1. Field of the Invention
The present invention relates generally to a method for detecting gradational zones in subterranean zones. More specifically, the present invention provides methods for detecting a gradational pressure transition zone and characteristics thereof prior to drilling therein by way of reflection seismology.
2. State of the Art
In the drilling of petroleum and other deep wells, one of the difficulties is in detecting when a drilling operation may be approaching or may be entering zones of increasing pressure. Unexpected entry into such “overpressured” zones can result in well blow-out, loss of equipment, injury and loss of life, and the release of oil and other fluids into the surface environment. During drilling of a borehole with a rotary drill bit, drilling fluids, usually referred to as “muds,” are circulated in the borehole to cool and lubricate the rotary drill bit, flush cuttings from the bottom of the hole, carry cuttings to the surface, and hydrostatically balance formation pressures encountered by the borehole. Mud weight is carefully monitored and may be increased during drilling operations to compensate for geopressure. In some areas, it may be necessary to install a casing within the borehole so that relatively heavy mud weights necessary to compensate for geopressure will not damage the borehole. Accordingly, attempts have been made to identify such overpressured zones by way of seismic analysis.
Seismic identification of formation characteristics is widely practiced by the energy industry as an exploration method in the search for hydrocarbons in the form of petroleum and natural gas. Typically, an input signal source at the ground surface is used to generate acoustic waves which radiate down through the earth, although the use of subsurface signal sources is also known. The waves may then be refracted and reflected by the stratigraphic layers in the subterranean formation. Particularly strong reflections may occur at boundaries between rock layers that have a marked difference in impedance caused by an abrupt change in velocity and density. The acoustic reflections may be detected and timed by a line of geophones on the surface, although disposition of geophones in a borehole is also practiced. In general, acoustic velocities are relatively slower in rocks with low elasticity magnitudes such as shales, but relatively faster in more rigid rocks such as sandstones or limestones. The reflections of the seismic record may therefore correspond to layers of differing rock types that may be used for stratigraphic work or the search for specific rock bodies.
There have been several attempts to predict the existence and location of overpressured subterranean zones from seismic reflection profiles. Velocity or density differences associated with the transition from normal formation pressures to overpressured zones sometimes may produce a sharp impedance boundary, causing a strong reflection. A synthetic seismogram may be generated from well logs in the vicinity of the seismic reflection profile to predict the amplitude and phase of the reflector corresponding to the top of the pressure gradient surface. Also, since Poisson's ratio differs for different pore pressures, pressure gradients may be located by analyzing seismic reflection profiles to determine the ratio of compressional velocity to shear velocity. Furthermore, compressional to shear velocity ratios (i.e., Poisson's ratio) may be produced from the use of both traditional shear seismic sources on land, or from predictive shear velocity logs based from logs of other physical properties.
Unfortunately, these methods are typically dependent on drilling a wellbore, performing logging operations to determine characteristics along the wellbore, and then analyzing the logging data to predict the existence of pressure gradients. Drilling into excessive pore pressure regions without forewarning may be dangerous and is generally undesirable. Accordingly, the prior art has attempted to predict gradational pressurized zones prior to drilling therein and without drilling a wellbore.
U.S. Pat. No. 5,311,484 to Anderson et al. describes a method of using reflection strength processing and redisplay to determine pressure gradient positions within a subterranean formation. Particularly, Anderson teaches that waveform smoothing of reflection strength traces may enhance the changes in the amplitude thereof, thus indicating geopressure.
However, a need clearly exists for a reliable technique of sensing the location of a gradational pressurized zone, or an acoustic gradient. Furthermore, a need also exists for a more reliable technique for locating deposits of natural gas and petroleum based on seismic analysis. Moreover, a need is evident for seismic-while-drilling measurement and prediction of gradational pressurized zones to prevent inadvertent drilling thereinto and subsequent possible blow out.